Seedling Development

Frank P. Ronco

Seedling Development

Natural regeneration is difficult to achieve, primarily because of unfavorable climatic conditions, but seed predation and heavy grazing pressure, especially by sheep and goats, also play a role (67). Although pinyon grows best in full sunlight and can germinate in the open, seedlings must be protected from the harsh environment (21,31,35,46,49). Regeneration is usually achieved in the shade of tree canopies, under shrubs such as rabbitbrush, mountain-mahogany, and sagebrush, or alongside fallen trees.

Optimum germination temperature for pinyon seed is about 21° C (70° F). Germination is epigeal (68). Preliminary studies indicate that germination can be significantly improved by washing seeds for 48 hours in running tap water. Cold stratification for 30 or 60 days increases speed of germination but not the percentage. Treatment with hydrogen peroxide to suppress mold and enhance germination generally is not effective. Seeds germinate in spring and summer following dispersal, depending on soil moisture and temperature, with summer germination coinciding with the onset of the rainy season. Also, seedling establishment probably depends on an adequate moisture supply during the first summer (29,30,54,66,68). Growth throughout the seedling stage is extremely slow, often with only primary needles developing the first year, and subsequent height growth averaging 2.5 to 5.0 cm (1 to 2 in) per year (67).